This invention relates to a liquid crystal display unit and, more particularly, to an active matrix liquid crystal display unit with spacers inserted between two substrate structures.
A typical example of the liquid crystal display is shown in FIG. 1. The prior art liquid crystal display unit is classified in an active matrix type with inverted stagger type thin film transistors. The prior art liquid crystal display comprises substrate structures, spacers 129 and liquid crystal 130. The spacers 129 are formed of transparent synthetic resin, and are like spherical beads. The spacers 129 are scattered over either substrate structure before the substrate structures are assembled. When the substrate structures are assembled, the spacers 129 create a gap between the two substrate structures, and the gap is filled with liquid crystal 130.
The lower substrate structure includes a glass substrate 101, scanning lines 102, thin film switching transistors 110, pixel electrodes 107 and an orientation layer 108. The thin film switching transistor 110 includes a part of the scanning line serving as a gate electrode, a gate insulating layer 103, a semi-conductor layer 104, a drain electrode 105 and a source electrode 106. The scanning lines 102 are patterned on the glass substrate 101, and are covered with the gate insulating layer. The semiconductor layers 104 are patterned on the gate insulating layer 104, and source/drain regions are formed in each of the semiconductor layers 104. The source region is spaced from the drain region, and the gate electrode or the part of the scanning line 102 is opposed to a back channel 111. The drain electrode 105 is held in contact with the drain region, and the source electrode 106 is held in contact with the source region. The pixel electrodes 107 are further patterned on the gate insulating layer 103, and are respectively associated with the thin film switching transistors 110. Each of the source electrodes 106 is connected to the associated pixel electrode 107. The thin film switching transistors 110 and the pixel electrodes 107 are covered with the orientation layer 108.
On the other hand, the upper substrate structure includes a glass substrate 121, a black matrix 122, a common electrode 123 and an orientation layer 128. The black matrix 122 is patterned on the glass substrate 121, and the common electrode 123 is patterned over the glass substrate 121 and the black matrix 122, and the black matrix is overlapped with a part of the common electrode 123. The common electrode 123 is covered with the orientation layer 128, and the orientation layer 128 is spaced from the orientation layer 108 by means of the spacers 129.
A problem is encountered in the prior art liquid crystal display unit in poor quality of images produced thereon. The poor quality images are due to the spacers 129 and obliquely incident light. In detail, the spacers 129 are scattered over one of the orientation layers 108/128 as described hereinbefore. It is unavoidable to position the spacers 129 over the pixel electrodes 107. The spacers over the pixel electrodes 107 vacate the liquid crystal 130, and permit the light to pass therethrough regardless of the orientation of liquid crystal molecules 130. Moreover, the spacers 129 vary the orientation of the liquid crystal molecules 130 therearound, and cause the amount of transmitted light and the tint to be uncontrollable. In case where the spacers 129 were not uniformly scattered, the amount of transmitted light is varied together with the dispersion of the density of the spacers 129. Thus, the spacers 129 are causative of the poor quality of images.
Although the black matrix 122 are formed on the glass substrate 121, the obliquely incident light reaches the back channel 111, and generates electron-hole pairs in the semiconductor layers 104. The electron-hole pairs vary the transistor characteristics of the thin film switching transistor 110, and potential difference is inappropriately applied between the associated pixel electrode 107 and the common electrode 123. This results in the poor quality of images.
A solution is disclosed in Japanese Patent Publication of Unexamined Application (laid-open) No. 8-234212. The prior art liquid crystal display is shown in FIG. 2. The prior art liquid crystal display also comprises substrate structures, spacers 169 and liquid crystal 170. A difference between the two prior art liquid crystal displays is the location of the spacers 169. The spacers 169 are not transparent. Although the spacers 129 are randomly scattered over the orientation layer 108/128, the spacers 169 are located over the back channel 151. When the substrate structures are assembled, the spacers 169 also create a gap between the two substrate structures, and the gap is filled with liquid crystal 170.
The lower substrate structure is similar to that shown in FIG. 1. Namely, the lower substrate structure includes a glass substrate 141, scanning lines 142, thin film switching transistors 150, pixel electrodes 147 and an orientation layer 148. The thin film switching transistor 150 includes a part of the scanning line serving as a gate electrode, a gate insulating layer 143, a semi-conductor layer 144, a drain electrode 145 and a source electrode 146. The scanning lines 142 are patterned on the glass substrate 141, and are covered with the gate insulating layer 143. The semiconductor layers 144 are patterned on the gate insulating layer 143, and source/drain regions are formed in each of the semiconductor layers 144. The source region is spaced from the drain region, and the gate electrode or the part of the scanning line 142 is opposed to the back channel 151. The drain electrode 145 is held in contact with the drain region, and the source electrode 146 is held in contact with the source region. The pixel electrodes 147 are further patterned on the gate insulating layer 143, and are respectively associated with the thin film switching transistors 150. Each of the source electrodes 146 is connected to the associated pixel electrode 147. The thin film switching transistors 150 and the pixel electrodes 147 are covered with the orientation layer 148.
The upper substrate structure includes a glass substrate 161, a common electrode 163 and an orientation layer 168. The common electrode 163 is patterned on the glass substrate 161, and is covered with the orientation layer 168. The spacers 169 are inverted between the orientation layers 148 and 168, and the gap is filled with the liquid crystal 170.
The spacers 129 are replaced with the spacers 169, and are non-transparent. Even if light is obliquely incident on the prior art liquid crystal display, the non-transparent spacers 169 prevent the back channels 151 from the incident light, and keeps the transistor characteristics constant. Moreover, there is not any spacer over the pixel electrodes 147, and the liquid crystal occupies the gap between the pixel electrodes 147 and the common electrode 163. This results in that the turbulence is negligible in the orientation of the liquid crystal molecules. The spacers 169 are less influential on the liquid crystal molecules between the pixel electrodes 147 and the common electrode 163. The orientation of liquid crystal molecules is simply dependent on the potential difference between the pixel electrodes 147 and the common electrode 163, and the transmittance of the liquid crystal 170 is constant in the liquid crystal 170 under a standard bias condition. Thus, the location of the spacers 169 is effective against the poor quality of images. However, a problem is encountered in the prior art liquid crystal display unit disclosed in the Japanese Patent Publication of Unexamined Application in that malfunction takes place in the thin film switching transistors 150.
In detail, although the spacer 169 is not connected to any source of voltage, the spacer 169 is charged and varied due to the electric field therearound. The protective insulating layer 148 is inserted between the spacer 169 and the semiconductor layer 144, and forms a capacitor together with the spacer 169 and the semiconductor layer 144. In other words, the spacer 169 serves as a back gate of a field effect transistor, and the potential level on the spacer 169 has an influence on the conductance of the back channel 151. The thin film switching transistor 150 is equivalent to the parallel combination of two field effect transistors as shown in FIG. 3. In FIG. 3, xe2x80x9cVSPxe2x80x9d, xe2x80x9cVGxe2x80x9d, xe2x80x9cVDxe2x80x9d, xe2x80x9cVCOMxe2x80x9d and xe2x80x9cCLSxe2x80x9d stand for the potential level on the spacer 169, the potential level on the scanning line 142, the potential level on the drain electrode 145, the potential level on the common electrode 163 and the capacitance of the liquid crystal 170, respectively. Even though the front channel 152 is controlled with the potential level VG, the potential level VSP on the spacer 169 is influential on the back gate 151 more than the potential level VG on the scanning line 142, and the spacer 169 gives the conductance to the back channel 151 depending upon the potential level VSP.
When the spacer 169 is charged, the electric charge changes the potential level on the spacer 169. Even if the spacer 169 is not charged, the spacer 169 is under the influence of the potential level on the common electrode 163, and the induced charge changes the potential level on the spacer 169.
FIG. 4 illustrates the voltage-to-current characteristics of a standard thin film transistor of amorphous silicon. If the spacer 169 does not have a sufficiently low potential level, current IDS flows through the back channel 151 in spite of the low potential level VGOFF, and undesirably changes the potential level on the pixel electrode 147. This results in that the liquid crystal molecules 170 change the orientation from the designed angle. When the orientation is changed, the piece of liquid crystal over the pixel electrode 147 changes the transmittance. If the potential level on the spacers 169 is dispersed, the lightness is undesirably varied over the images. This results in poor quality of images produced on the prior art liquid crystal display unit.
It is therefore an important object of the present invention to provide a liquid crystal display unit, which produces high quality images.
To accomplish the object, the present invention proposes to bias a spacer to a certain voltage level so as to decrease a conductance of a back channel.
In accordance with one aspect of the present invention, there is provided a liquid crystal display unit comprising a first substrate structure including a first transparent substrate, at least one thin film transistor fabricated on the first transparent substrate and having a first channel region and a second channel region closer to the first transparent substrate than the first channel region, at least one pixel electrode formed over the first transparent substrate and connected through the at least one thin film transistor to a source of signal for creating an electric field and a protective insulating layer covering the at least one thin film transistor, a second substrate structure spaced from the first substrate structure so as to form a gap, at least one conductive spacer inserted between the protective insulating layer and the second substrate structure and connected to a source of potential level so that the first channel region is biased with a certain potential level for decreasing a conductance of the first channel region, and liquid crystal filling the gap between the protective insulating layer and the second substrate structure and changing an orientation depending upon the strength of the electric field.